Liquefaction of air



I H. N. DAVIS;

L|QUE FACTION OF AIR. APPLICATION mzo APR-24, 92o.

PatentedJune 27, 19.22.

I Qua/new UNITED PATENT OFFICE.

HARVEY N. DAVIS, 01 CAMBRIDGE, MASSACHUSETTS, ASSIGNOR '10 RESEARCH COR PORATION, OF NEW YORK, N. Y.,

A CORPORATION OF NEW YORK.

LIQUEIEACTION OF AIR.

Specification of Letters Patent. Patented June 27, 1922,

Application filed April 24, 1920. Serial No. 376,408.

' mixtures rich in oxygen or nitrogen, from the atmosphere, and more particularly to those parts .of such apparatus which are commonly called fore-coolers or interchangers or regenerators or liquefiers.

It is the general object of the invention to make processes and apparatus of the said kind more efiicient, to increase the yield, to reduce the power required to efiect the separation of a given amount of useful product and to decrease the cost of the use ful product, which may be either oxygen or nitrogen of any useful degree of purity or both.

For a full understanding of the invention reference is had to accompanying drawing which is a diagrammatic view of an arrangement embodying the invention.

Before describing the specific embodiment of the invention I desire to explain certain underlying fundamental principles.

The purpose of a heat interchanging system in apparatus of the type under consideration is to receive at one end one or more streams of warm fluid on their way to the rectification apparatus and to receive at the other end one or more streams of cold fluid on their way from the rectification apparatus, still or separating apparatus in general, and to cause the cold streams to absorb heat from the warm streams in the manner known to engineers as counter-cur-- For efiicient operation it is also necessary,

as is well known. that the second set of streams should leave the interchanging system at as high a temperature as possible. In what follows, the first set of streams which 'are losing heat, will be called the downgoing streams and the second set of streams, wh1ch are receiving heat, will be called the upgoing streams.

The invention which is closely interlinked.

with the subject matter of my co-pending applications Ser. No. 394,544 filed July 7, 1920 and Serial'No. 424,760 filed Nov. 17,

1920 is based on two fundamental principles I. The sum of the effective heat transfer capacities of all the downgoing streams in an interchanger should be made as nearly as possible equal to the sum of the effective heat transfer capacities of all the upgoing streams at every level. v

II. All necessary heat pumping should be done with the least possible expenditure of work.

There are well known thermodynamic reasons why this interchange of heat should be effected in such a way that the streams receiving each B. T. 'U. of heat should be as nearlyas possible at the same temperature as the streams giving up that same B. T. U. of heat. This requires that all the streams passing each level in the interchanger shouldbe. as nearly as possible at the same temperature, and this is possible only when all the up-going streams taken together have at each level the same, or nearly the same, effective heat transfer capacity as all the down-going streams at that level, taken together. I It is obvious from the law of the con servation of matter that the total quantity of matter to be warmed up by the interchanging system, including the expansion engines and expansion valves, is the same as the total quantity of matter to be cooled down, if, as is usually the case in apparatus of the type under consideration, no matter is withdrawn from or introduced into the colder parts of the apparatus. But this potential equality of matter in the downgoing and upgoing sets of streams does not lead to equality in the effective heat capacities of the two sets of streams, for two reasons.

Inthe first place a certain amount of heat will inevitably leak into the interchanging system itself, from its surroundings, throu h the insulation or lagging, and some heat wi inevitably leak from warmer to cooler parts of the system by conduction. Each of these kinds of heat leak decreases the amount of of down going streams by a given set'of up going streams, andhasthe same effect as would be produced if the down going streams in a perfect interchanger had a greater heat absorbing capacity or specific heat than the up-going streams.

And in the second place, certain of the down going streams must always be at higher pressures than anyof the upgoing streams,-

at least in apparatus of the type under consideration, because a certain amount of heat must always be pumped out of thejseparating or rectifying apparatus, both to remove heat that has leaked in andto accomplish the desired separation; But the'specific heat of a gas at high'pressure is known to be greater than the specific heat of the same or an equivalent gas at low pressure. There is, therefore, a second, and in this case an actual as distinguished from an effective excess in the heat transferring capacity of the down going streams as compared with the up going streams.

.To accomplish a thermodynamically eflicient thermal exchange between these sets of streams, it is therefore necessary either to increasethe effective heat capacity of the up going streams, or to decrease the effective heat capacity of the down going streams. This has been done in a more or less imperfect fashion in several of the air liquefaction and distillation systems already known to the art, but the method about to be described' allows the operator of the'apparatus to accomplish the desired adjustment of the effective heat capacities of the two sets of streams with greater nicety than has hitherto been possible.

It consists in by-passing successive portions of the set of down going streams through a plurality of expansion engines or turbines of which three are represented in the drawing namely 8, 9 and 10, thereby decreasing the uantity and so the efi'ective heat capacity 0 that portion of-the set of downgoing streams that remains in the interchanging system. The first of this series of engines should withdraw a suitable portion from'the set of down going streams at a point at or near the warm end of the interchanging system, should cool it as much as possible, both by withdrawing energy from it in the form of mechanical'energy, work, or power, and by means of the J oule Ihompson effect, and should return it to the interchanging system at a point where the temperature in the interchanging system is as nearly as possible equal to that at which the stream is discharged from the engine.- The last of the series of engines should withdraw a suitable portion from the set of down going streams at such a point, well down the interchanging system, that the tempera.- ture of the exhaust from this engine will be of the exhaust itself.

substantially. equal to, or lower than, the temperature at the colder end of the interchanger, and should return this exhaust at a point in the apparatus, either in the interchanger or in the column or still, where the temperature is substantially equal to that of the exhaust itself. The intermediate engine or engines of the series, if there be more than two, should withdraw portions of the set of down going streams at points intermediate between the intake points of the first and last engines of theseries and each exhaust should be returned to the interchanger or still ata point where the temperature is substantially the same as that In the drawing, the temperature ranges covered by the three engines, 8, 9 and 10 are represented as being approximately consecutive, each point of withdrawal being, for clearness in the drawing, represented as close to the point at which the preceding exhaust is returned to the interchanger, but it is understood that the scope of myinvention is not limited to this particular arrangement, but that, particularly in large installations, the number and arrangement of the expansion engines may be such that two or more of their temperature ranges overlap, and furthermore that, particularly in small installations, the number and arrangement of the expansion engines may be such asto leave one or more gaps between their temperature ranges. Y

Furthermore, one of the series of engines is represented as fed by a high pressure nitrogen stream, and two by a high pressure air stream, but it is understood that the scope of my invention is not limited to this particular arrangement. Any arrangement which by-passes two or more temperature intervals for two or more portions of the set of down going streams, by expanding these portions through engines or turbines, in such a way as to give an approximate realization of my ideal of equalizing the effective heat capacities of the two sets of streams at each and every level in the interchanging system is considered as within th scope of my invention.

A second feature in the dispositions of apparatus to be heneinafter described is dictated by this same fundamental principle I. I have already mentioned the necessity of pumping heat out of the column or still, or, as it is more commonly stated, of maintaining liquid in the column or still. In the present state of the art (see for example U. S. Patent #967,104) this is often done by passing through the interchanger system 'a stream of fluid at a pressure below its critical pressure, but above that of the up going streams. In what follows pressures of this order of magnitude will be called intermediate pressures (I. R), as disdifliculty by making the pressure of the I.

tinguished on the one hand from pressures not much above atmospheric, which will be called low pressures (L. P.)', and, on the other hand, from pressures well above the critical'pres'sures of the fluids in question, which will be called high, pressures (H. P.). The result of passing I. P. fluid through the interchangersystem is to liquefy a part or all of it in the colder parts of the system, which parts are often called by separate names such as liquefiers, etc. Now this liquefaction ordinarily involves the giving up of a considerable amount of latent heat with yery little corresponding fall'in temperature, and has the same effect as a very large specific heat in making it difficult to keep the heat-transfer capacities of the two sets of streams substantially equal at every level in this part of the interchanger. Some have sought to avoid this P. streams with which they were dealing substantially equal to or even slightly greater than the critical pressures of the fluids in question, thus as they claim, avoiding latent: heat altogether. But this procedure merely alters theform in which the difiiculty in question appears, for it is well known that the specific heat at constant pressure of any fluid is abnormally high over a small temperature range in the neighborhood of its critical temperature, even when the pressure is kept somewhat above critical so that ,no liquefaction can occur. I therefore propose that, whenever i it is desirable to pass fluid through an interchanging system under pressure, so as to use such fluid for umping heat out of a column or still, this fluid should, if possible, be what I have defined above as H. P. fluid,'that--isa' fluid atpres'sure well above its critical pressure where abnormally high specific heats are not concentrated in any one comparatively limited temperature range, and not whatI have defined above as I. P. fluid, that is a fluid at a pressure below, at, or close to its critical pressure where either latent heats; or abnormally high specific heats do appear within a com-.

paratively limited temperature range.

\ Whenever, as is sometimes the case, it is desirable to supply I. as distinguished from H. P. fluid to a" column or still for specific purposes to Which H. P. fluid is not appropriate, the required I. P. fluid should be formed, if possible, either by throttling H. P. fluid after it leaves the interchanger system, or in a manner to be explained hereinafter. 1

We now turn to the proposition that all necessary heat pumping should be done with the least possible expenditure of work.

' In previous apparatus of the type under consideration, and, in the apparatus proposed in these specifications, the pumping of heat is accomplished by putting a certain quantity of fluid into the system under pressure, and withdrawing the same or an equivalent quantity of fluid under a lower can theoretically pump out of the system a quantity of heat given' by the integral 5} cap,

where "u is the Joule- Thompson coeflicient, C is the specific heat, 1),, is the lower pressure, p is the higher pressure, and the integration is at constant temperature, and

at that temperature at which the fluid enters the system, which is ordinarily determined largely by thecooling water available. -It is also an established thermodynamic fact that if the said fluid, in expanding somewhere in the system from the higher pressure p to-the lower pressure 12 can be I made to perform external work, which work is removed as work from the system to some wholly external work absorbing ap-' paratus, then the said'fluid can also pump out of the system an additional amount of heat equal to the heat value of the work removed from the system. The fact that .omy of the Joule-Thompson heat-pumping effect are, as is well known, muchincreased by putting the air tube expanded under" an initial pressure that is materially abovethe critical pressure of air, and preferably to a pressure above 2000 pounds per square inch, or above about three and a half times the said critical pressure. .But those who have, made use of this fact, have apparently not realized that if this high pressure air is expanded in an engine or turbine there is a clear gain in its heat-pumping power equal to the whole amount by which the work removed from the system exceeds the increase in heat leak due to the presence of the expansion engine itself, which is not in general serious.

On the other hand, those who have made use of expansion with external work, have ordinarily compressed a large amount of air to a comparatively low pressure, less than or at most substantially equal to its crltlcal' '95 these'two kinds of heat pumping powers may be both in action and additive, seemsa larger heat-pumping efl'ect, because the of the theoretically possible J oule-Thomp-w son heat pumping power can be made effective in practicable apparatus, while only a part of the theoretically possible expansionengine heat-pumping power can be made practically effective, because of the unavoidable inefiiciency of any form of expansionengine.

' A further consequence of the fundamental principles mentioned above is that, wheneverwhat I have called I. P. air is required in the column or still, it can be most advantageously supplied by feeding what I have called H. P. air through the interchanging system" as far as the intake'of the coldest ei pansionengine, and there expanding it,

t not to atmospheric pressure, but to the intermediate pressure desiredl By this means we secure the heat-pumping power of this air over a pressure range lying largely above its critical pressure, where the ratio of heatpumping power to work of compression is somewhat higherthan at lower pressures,

because of the efi'ectiveness of the J oule- Thompson heat-pumping effect at high pressures.

Of course, if the amount of air which it J is advantageous to expand in the coldest engine is not enough to meet the requirements of the column or still, supplementary air can m 0 be carried at high pressure through all, or

an appropriate part, of the interchanging system and then throttled into the exhaust line from the said engine.

A third possibility of reducing the work 4 thatmust be expended to secure the desired amount of heat-pumping arises when, as is sometimes thecase, it is desirable to feed a stream'of pure nitrogen into the column or still. Under these conditions, if the required amount of nitrogen is suflicient, it should be put under what I have called a high pressure and cooled in the interchanging system to such a temperature that when it isv expanded, with the performance of external used in'the column or still, its temperature is appropriate for immediate entry into the still, without further cooling in the ,interchanger. This procedure has all the advanceding paragraph, and the additional ad- 7 working substance at low temperatures than air.

' Having now described the fundamental Q5 vantage that nitrogen is a slightly better train contained in a shell or column 2'.

work, to the pressure at which it is to be gine, the engine 10, is connected to the pastages of the. procedure described in the pre-' principles on which the invention is based, the nature of the invention will be easier understood. I v I Having reference to the drawing, 1 represents an interchanger and 2 a rectifica lii pln interchanger carries, in the'particular embodiment, three downgoing streams composed respectively of L. P. air, H. P. air and H. P..-

nitrogen, and two up going streams of prodnot from the column or separating device. For the sake of simplicity the interchanger is represented. as being composed of five tubes or passages 3, 4, 5, 6 and 7 respectively for bringing the'streams in heat-interchanging relation.

Tube 3 is in practice connected at its warm end to a compressor (not shown) operating with a *pressure just. suflicient to cause flow of air through the interchanger into the separating device.

Tube 4 is in practice connected to a compressor or compressor system adapted to produce high ressure .such as above! referredtoasI-I.

Tube 5 is similarly connected to a P. compressor system by which the nitrogen is compressed to H. Pfs .such as previously referred to.

Tubes 6 and 7 are connected at their cold ends to the separating device-2 and at their warm ends to suitable receivers (not shown), as is well understood.

Three expansion engines, 8, 9 and 10' are providedin accordance with the principles stated above. Two of these engines are connected at the intake side to the passage 4;, containing the H. P. air and at the exhaust side to the assage 3 containin he low pressure air. Engine 8 is fed H. air from a point near the warm end and discharges the exhaust at a point along the heat-exchange train colder than the polnt at which the air is withdrawn. Engine 9 is fed H. P. air from a point along the heat-exchange train colder than the intake point of engine 8- and the exhaust of engine 9 is returned to pamge 3. at a point colder than, the point of withdrawal. It is understood that any plurality. of expansion engines may be employed and that the points of withdrawal and return may be chosen in various ways so as to suit the particular conditions.

In the embodiment shown, the coldest ensage 5 containing the H. P. nitrogen at a point in the heat-exchange train relatively close to the cold end thereof and exhausts I. P. nitrogen which maybe usedv in the still or separating device in a well known manner.

The use of the fluids pawing out of the cold end of thetubes 3, 4 and 5 and the ex- .haust from the nitrogen engine 10 is fully described in' my rnependin application above referred to. It is s dient for the 30 purposes of the present disclosure to state that the L. P. gaseous air is introduced at the level 18 of the still and that the H. P. air is throttled, by means of the throttle 32, into the still at the level 31 after being so cooled as to liquefy-almost completely at the throttle. Each of these streams is put in thermal contact with the contents of the lower run of the still up to the low pressure air inlet level 18 by being passed through coils 20 and 30 respectively Additional heating power for the lower run and additional cooling power for the lower portions of the other two runs are provided by carrying a part of the H. P. nitrogen steam through the whole of the interchanger, and thence by the pipe (12) to the coils (33) in the lower run. This stream is then represented as being divided and throttled. through the valves (38) and (39) into the coils (36) and (37), in the lower portions of the two upper runs. The remaining portion, if any, of this H. P. nitrogen stream is throttled through the valve (40) into the top of the'main train.

The nitrogen product from the top of the still, and the nitrogen gas discharged from the coils (36) and (37) are carried through the pipes and coils (14), ('15) and (16) to the bottom of the rectification train, being thereby warmed substantially to the temperature appropriate to the bottom of the interchanger, and being thereby caused to yield to the contents of the main train all of their available cooling power, which is thus used for the purposes described above, and to assist in keeping the train cold in the face of the various inevitable heat leaks The oxygen liquid leaving the main rectification train at the bottom thereof is deflected by the diaphragm (21) into the well (22) where it comes into thermal contact with a part of the I. P. nitrogen in the coil (23) and is subjectedto a final purification by boiling, the intensity of which is relgulated by one of the hand-valves (27). he liquid oxygen then passes through the trap (24) into the outer boiling-ofi' pool (25) where it is vaporized by heat exchange with the I. P. nitrogen in the coils (26). The gaseous oxygen product then leaves the still through the pipe (6). In themeantime the largely or wholly liquefied I. P. nitrogen from the coils (23) and (26) is throttled into the to I of the main train through the xzalxses (27 the riser (28) and the valve The arrangement of the diaphragm (21) the inner and outer boiling ofi' pools (22) and (25), the coils (23) and (26) and the valves (27) has a considerable advantage in that the oxygen product is kept entirely separate from the gas fed into the lower part of the rectification train, which latter portion is caused to contain the larger part of any nitrogen or other volatile impurity that reaches the bottomof the train. Meanwhile complete control over the relative quantities of the two portions of gas evaporated from the oxygen rich liquid is afforded by the valves (27). a

\Vhile I have shown an IQP. fluid as derived from the exhaust of the nitrogen engine I also contemplatethe use of I. P.-,

fluids derived from the exhausts of engines fed from the H. P. air passages.

For convenience I have indicated only a single H. P. air stream, but I contemplate the use of a plurality of H. P.'streams.

I therefore do not intend to be limited to the particular embodiment shown in the drawing.

1. In a process of effecting heat exchange between a gaseous stream and a relatively colder gaseous stream flowing in countercurrent heat interchanging relation, the step of promoting equality between the effective I heat-transferring capacity of the warmer stream and that of the colder stream, which consists in withdrawing from the warmer stream a portion of the gas, coolingit and returning the cooled portion to counter-current heat interchanging relation with the said relatively colder gaseous stream at a point colder than its point of withdrawal.

2. In a process of efl'ecting a heat exchange between gaseous streams and relatively colder gaseous streams flowing in counter-current heat interchanging relation, the step of promoting equality between the effective heat-transferring capacity of the warmer streams and that of the colder streams which consists in withdrawing from.

a warmer stream a portion of the gas and returning it to counter-current relatlon wlth the said relatively colder gaseous streams at a point colder than its point of withdrawal consists in withdrawing from a warmer stream a portion of the gas,causing it to expand with the performance of external work and returning the expanded gas to counter-current heat interchanglng relation with the said relatively colder gaseous.

streams at a point colder than its .point of withdrawal.

4. In a process of effecting a heat exchange between gaseous streams and relatively colder gaseous streams flowing in countercurrent heat interchanging relation, the'step of promoting equality between the effective heat-transferring capacity of the warmer streams andthat of the colder streams, which consists in withdrawing from a warmer stream, at points of difierent temperature conditions, portions of the gas and change between gaseous streams and rela-.

returning the withdrawn portions to heat interchanging relation at points colder than their respective points of withdrawal.

5. In a process of efiecting a heat .ex-

tively colder gaseous streams flowing in countercurrent heat interchanging relation,

the step of promoting equality between the efiective heat-transferring capacity of the warmer streams and that of the colder streams, which consists in withdrawing from a warmer stream, atpoints of difl'erent temperature conditions, portions of the gas, causing the withdrawn portions to expand with the performance of. external work and returning the expanded portions to heat-in: terchanging relation at points colder than their respective points of withdrawal.

6. In; a process of effecting a heat-exchange between gaseous streams and relatively colder gaseous streams flowing in -.cooling the withdrawn portions and returning them to the train at points colder than their respective points of withdrawal.

7. The process of pre-conditioning gases for liquefaction by putting them in countercurrent heat-exchanging relation with cold gases, the method which consists in putting portions, of the gases that arebeing cooled under apressure in excess of that'at which the cooled gases are to .be'used, withdrawing such portions, from heat-interchanging rela tion at difi'erent temperature levels andiex pending them, with the performance of external work, to the pressure at which they are to be used. I

r 8. The process of pre-conditioning gaseous I mixtures for treatment in a separating device, which consists in passing a stream of the mixture under a pressure materially higher than its critical pressure through a heat-interchanger in counter-current heat interchanging relation with cold gases, withdrawlng om the stream a portion ofthe f mixture, cooling it and returning the cooled portion to counter-current heat interchang- 1,412o,eae

ing relation with the said cold gases at a point colder than its point of withdrawal.

9. The process of pre-conditioning gaseous mixtures for treatment in a separating device, which consists in passing a stream of the mixture tinder a pressure materially higher than its critical pressure through aheat-interchanger in counter-current heat interchanging relation with cold gases, withdrawing from the stream, at points of different temperature conditions, portions of the mixture, cooling the said portions and bringing them into heat-interchanging relation with the other streams of the interchanger at points colder than their respective points of withdrawal.

10. The process of pre-conditioninggas- I eous mixtures for treatment in a separating device, which consists inpassing a stream of of the mixture under a pressurematerially higher than its critical pressui e through a heat-interchanger in countercurrent heat interchanging relationwith cold gases,withdrawing from the stream, at points of different temperature conditions, portions of the mixture, causing the withdrawn portions to expand with the performance of external work and bringing the expanded portions into heat-interchanging relation with the other streams of the interchan er'at points colder than their respective polnts of withdrawal. I 11. The process of pre-conditioning gas eous mixturesfor treatment in a separating device, which consists in passing a stream of the mixture under a pressure materially above the critical pressure and a stream of the mixture under low pressure through a heat-interchanger in counter-current heatinterchan ing relation with cold gases, withdrawing points of diii'erent temperature conditions, portions of the mixture, expanding the withdrawn portions against external work and passing the expanded portions'into the mm the high-pressure stream, at

low-pressure stream in heat-interchanging' relation with the-other streams at points colder than their respective points of withdrawal. v v

12. Process for pie-conditioning gaseous mlxtures for treatment in a separatlng device, which consists in forming one or more H. P. streams and one or more L. P. streams of the mixture to be treated, putting all of said streams in heat-exchanging relation with cold products of the treatment,-with- 1 drawing from the H. P. fluid portions at difierent temperature levels, expanding them with the performance of external work and returning each expanded portion to the apparatus at a point at which the temperature of the mixture to be treated is'substantially equal to the temperature of the-expanded portion. v

for efiecting a heat ex- 130 13. Apparatus change between a gaseous stream and a relatively colder gaseous stream, comprising means for bringing the two streams in countercurrent heat-interchanging relation, sald means including means for putting at least tercurrent heat-interchanging relation, means for putting different parts of the gases to be cooled under different pressures, means for withdrawing at different temperature levels portions of gas under higher pressure, means for expanding the withdrawn portions with the performance of external work and means for returning each of the expanded portions to the heat-exchanging train at a point colder than that of its withdrawal.

15. Apparatus for effecting a heat exchange between warm and relatively cold gaseous fluids, comprising means for bringmg the two fluids in countercurrent heatinterchanging relation, means for putting different parts of the gases to be cooled under different pressure, means for withdrawing portions of gas under higher pressure at different points along the heat-exchanging train, means for expanding the withdrawn tions with the performance of external workand means for returning the expanded portions to different points along the train at points colder than their respective points of withdrawal.

17. Apparatus'for conditioning air, comprising a conduit and means for passing cold gases therethrough, asecond conduit and means for pming air to be treated therethrough under relatively low-@pressure in counter-current heat-interchanging relation with the cold gases, a third conduit and through under a higher pressure also in counter-current heat-interchanging relation with the said cold gases, and means for expanding air from a warmer level of the higher pressure conduit to a colder level of the lower pressure air conduit.

18. Apparatus for conditioning air, comprising a conduit and means for passing cold gases therethrough, a second conduit and means for passing. air to be treated therethrough under relatively low pressure in counter-current heat-interchanging relation with the coldgases, a third conduit and means for passing air to be treated therethrough under a higher pressure also in counter-current heat-interchanging relation with the said cold gases, and means including an expansion engine for expanding air with the performance of external work, from a warmer level of the higher pressure air conduit to a colder level of the lower pressure air conduit. 1

19. -In apparatus for conditioning air, a device for bringing cold gases. in cou'ntercurrent heat-exchanging relation 'with the air to be treated, said device including a passage for air under relatively low pressure, a pas- Sage for air under a higher press'ure,.and a plurality of means for expanding air from different relatively warmer levels of the higher pressure air passagev to relatively colder levels of the lower pressure air passage. c

20. In apparatus for conditioning air, a devicevfor bringing cold gases in countercur-- rent heat-exchanging relation with the air to be treated, said device including a passage means for passing air to be treated there-..

for air under relatively low pressure, a pas-' Sage for air under a higher pressure, and a plurality of means, each including an ex pansion engine, for expanding air with the performance of external work from differ ent relatively warmer levels of the higher pressure air passage to relatively colder levelsof the lower pressure air passage.

21. In apparatus for separating air into .plurality of portions of different compositions, a rectification train, a device for bringing cold gases from the said train into countercurrent heat-exchanging relation with air to be treated, said deviceincluding al passage for air under relatively low pressure, a passage for air under a higher prestwoor more portions of different composi tions by rectification, a rectification train, a

device for bringing cold gases from the train into countercurrent heat-exchanging. relation with gases. being conditioned for passage to the separation apparatus, said device includin a passage for low-pressure air, a

passage or air under a higher pressure, and

a passage for previously purified nitrogenrich' gas, means lncluding an expanslon englne for expandmg a1r from'one or more relatively warmer levels of the higher. pressure air passage to relatively colder levels of' the low pressure air passage and means including an expansion engine connected with the nltrogen passage for expanding a part orall of the nitrogen-rich gas.

24%. A heat-interchanger containing a passage for cold gases, a passage for a gaseous the performance of externalwork.

neameaa mixture to be cooled, means for passing the mixture through its passage under a high pressure, and means operative to equalize at different levels of the interchanger the heat-transfer capacities of the gases in the two passages, saidmeans including means for withdrawing from the high-pressure passage, at points of different temperature conditions, portions'of the mixture, cooling said portions and bringing them into heatinterchanging relation with the gas passages of the interchanger at points colder than their respective points ofwithdrawal.

25. In a process ofefi'e'cting heatexchange between a gaseous stream and a relatively colder gaseous stream flowing in countercurrent'heat interchanging relation, the step of promoting equality between the effective heat transferring capacity of the' warmer stream and that of the colder stream which consists in withdrawing from the warmer stream successively, at a plurality of levels 'each colder than .the prweding portions of the warmer stream, and cooling each such portion, independently by expansion with In testimony whereof, I aflix my signature.

HARVEY DAVISE 

